1. Field of the Invention
The present invention generally relates to a camera and, more particularly, to an active type distance measuring device for auto-focusing in a camera which radiates distance measurement light on an object to be photographed, and detects the object distance in accordance with the reflected signal light of the distance measurement light.
2. Description of the Related Art
Conventionally, an active trigonometric distance measuring device which radiates light such as infrared light toward an object to be photographed, receives a signal light beam reflected by the object via a light-receiving lens separated from a light-emitting means by a base length, and detects its incident position to detect the object distance is known as a distance measuring device for auto-focusing used for a camera.
It is well-known that the light beam incident position is detected by an optical position sensing device (PSD) constituted by a semiconductor, and it is described in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 50-23247.
The present applicant proposes a distance measuring device which performs distance measurement at different distances upon switching, and calculates not only a light beam incident position but also an amount of incident light beam in accordance with an output signal from a PSD to calculate the object distance, according to Jpn. Pat. Appln. KOKAI Publication No. 1-291111.
However, it is difficult to perform distance measurement with respect to an object in a long distance range with the above active trigonometric distance measuring device which uses a PSD for distance measurement. That is, an amount of incident signal light beam reflected by the object at a long distance is at a very low level of about 1 nA or 2 nA, and as a matter of course, an output current from the PSD is 1 nA or less. For this reason, noise produced from a circuit in amplification calculation of the signal tends to be superposed.
As described in Jpn. Pat. Appln. KOKAI Publication No. 1-291111, the present applicant proposes a distance measuring device using an amount of signal light beam reflected by the remote object. In this scheme, however, distance measurement is affected by the reflectance corresponding to a color of the object, and even if an object, e.g., a person in a white dress and an object, e.g., a person in a black dress position at the same distance, distance measurement results tend to be different.